1. Field of the Invention
The present invention relates to a drive circuit for a luminous element in a display device, and specifically relates to a drive circuit for a display device appropriate for driving a current-controlled luminous element such as organic and inorganic EL (Electro Luminescence) elements and an LED (Light Emission Diode) whose luminance is controlled by a current flowing through it.
2. Description of the Related Art
A display device where scan lines and signal lines form a matrix, and luminous elements such as organic and inorganic EL elements and LEDs are provided individual intersections of the scan lines and the signal lines to display a character as a dot matrix is widely used for a television set, a portable terminal, and an advertising board. Especially, since the elements constituting the pixels are luminous elements, this type of display devices do not require a back light for illumination while a liquid crystal display device requires it, have characteristics such as a wide view angle, and thus are attracting attention. Especially, an active drive display device, which includes switching elements integrated into the individual pixels on the matrix, and holds an image represented by the pixels for a certain period, has characteristics such as higher luminance, higher resolution, and lower power consumption compared with a passive drive display device which includes only luminous elements, and thus is especially attracting attention recently.
For this type of display device, conventionally a drive circuit shown in FIG. 1 has been used generally. In this conventional drive circuit, a scan line 201 turns on a switching transistor Tr201, a voltage on the data line 202 is written to a hold capacitor C202, and then the drive transistor Tr202 is turned on. A current corresponding to conductivity determined by the gate-source voltage of the drive transistor Tr202 flows through an EL element 200. Namely, the voltage of the data line 202 conducts analog control of gradation display. However, since the channel in a polysilicon thin film transistor used for the active drive display device is polycrystal silicon, variation of the characteristics is remarkably large compared with single crystal silicon. Thus, when the same gate voltage is written, the current varies depending on the pixels due to the variation of the characteristics of the drive transistor Tr202, a luminance becomes uneven, and consequently high gradation display becomes difficult. To overcome this defect, a drive circuit which is not affected by variation in a threshold voltage is disclosed on pages 438 to 441 by Sarnoff Corp. in xe2x80x9cSID 99 DIGESTxe2x80x9d in 1998 published by Society for Information Display.
The following will describe the operation thereof while referring to FIG. 2 and FIG. 3.
All of thin film transistors (Tr101 to Tr104) are constituted by P-channel transistors. In a period {circle around (1)}, all of the transistors Tr101 to Tr104 are turned on, and a current flows through an EL element 100. In a period {circle around (2)}, the transistor Tr104 turns off, a current flows on a path indicated by an arrow until the gate-source voltage Vgs of the transistor Tr102 reaches a threshold voltage Vth, and the transistor Tr102 turns off when Vgs=Vth. In a period {circle around (3)}, the transistor Tr103 turns off, and the voltage on a data line 102 changes VDD to Vdata. Then, the voltage generated between the both ends of the capacitor C102, namely the gate-source voltage Vgs of the transistor Tr102, becomes xe2x88x92VDD+Vth+C101xc2x7(VDDxe2x88x92Vdata)/(C101+C102). In a period {circle around (4)}, when the transistor Tr104 turns on, current I flowing through the EL element 100 is (Wxc2x7uxc2x7Cox/2xc2x7L)xc2x7((xe2x88x92C102xc2x7VDDxe2x88x92C101xc2x7Vdata)/(C101+C102))2 if the transistor Tr102 is used in the saturation region. Since this expression does not include the threshold voltage Vth, even if there is a variation in Vth, the current is not affected. Here, xe2x80x9cLxe2x80x9d and xe2x80x9cWxe2x80x9d respectively indicate channel length and channel width of the transistor Tr102, xe2x80x9cuxe2x80x9d is mobility, and xe2x80x9cCoxxe2x80x9d is gate dielectric film capacitance.
However, in this drive circuit, as the equation for calculating the current I described above clearly shows, though the variation of the threshold of the transistor can be compensated, the mobility of the transistor cannot be compensated. Thus, when there is a variation in the mobility, the luminance of the individual pixels fluctuates, and unevenness in the luminance occurs. Also, since this drive circuit requires two control liens in addition to the four transistors, the two capacitors, the scan line, and the data line, a pixel circuit becomes complicated, and the following two problems also occur.
The first problem is that probability of defects in production increases due to the complicated pixel circuit, and thus the yield decreases.
The second problem is that it is necessary to increase the current to provide intended luminance due to decrease of aperture ratio, and thus the power consumption increases.
An object of the present invention is to provide a drive circuit for a display device which does not present a luminance unevenness even when there is a variation in characteristics of a transistor, and to provide a drive circuit for a display device enabling a high gradation display.
In addition, another object of the present invention is to provide a drive circuit for a display device which prevents decrease in the yield and the aperture ratio, and decreases the price and the power consumption by simplifying the constitution of a pixel circuit.
A drive circuit for a display device according to the present invention is a drive circuit for use in a display device with a plurality of pixels arranged as a matrix and luminous elements being provided for the individual pixels. The drive circuit comprises:
drive transistors provided for the individual luminous elements and driving said luminous elements, said luminous element and said drive transistor in each of the pixels being serially provided between a first power supply and a second power supply;
a first switching transistor provided in each of the pixels for supplying a gate of said drive transistor with a control signal for controlling said drive transistor; and
a differential amplifier for comparing a voltage of a connection point between said luminous element and said drive transistor in each of said pixels, and a control voltage input in said differential amplifier and indicating luminance of the pixel, and, thereby generating said control signal, wherein
said control signal is supplied for the gate of said drive transistor through said first switching transistor.
In this drive circuit for a display device, as another aspect of the present invention, a second switching transistor may supplies said differential amplifier with said voltage of said connection point between said luminous element and said drive transistor in each of said pixels.
Also, both of said first switching transistor and said second switching transistor may be controlled by the same second control signal.
Said drive circuit for driving a display device may comprise a hold capacitor holding a voltage between the gate and the source of said drive transistor.
As another aspect of the present invention, a circuit for canceling an input offset may be provided for the differential amplifier.
As another aspect of the present invention, the differential amplifier may be formed on the same substrate as the pixel.
In addition to these constitutions, it is possible to further constitute such that the control voltage which is supplied for the display device, and indicates the luminance of the pixel is applied to the inverted input terminal (xe2x88x92) of the differential amplifier, and simultaneously, the voltage between the luminous element and the drive transistor is applied to the non-inverted input terminal (+) of the differential amplifier.
Since the present invention is constituted as described above, the first and the second switching transistors are turned on while a pixel is selected, and thus a feed back loop is formed by the differential amplifier. As a result, the gate of the drive transistor is driven such that the voltage of the image signal indicating the luminance information of the pixel and the voltage impressed on the luminous element are the same. Thus, even when there is a variation in the characteristics of the drive transistors, a variation does not present in the currents flowing through the luminous elements, and the uniformity of the display increases consequently.